CHAPTER 1 Failures due to long-term behaviour of heavy structures L. Binda, A. Anzani & A. Saisi Department of Structural Engineering, Politecnico di Milano, Milan, Italy. 1.1 Introduction The authors’ interest towards the long-term behaviour of heavy masonry struc- tures started after the collapse of the Civic Tower of Pavia in 1989, when L. Binda was involved in the Committee of experts supporting the Prosecutor in the trial, which involved the Municipality and the Cultural Heritage Superintendent after four people died under the debris of the tower. The response required by the Committee concerned the cause of the failure; therefore an extensive experimental investigation on site, in the laboratory and in the archives was carried out and the answer was given within the time of nine months. Several hypotheses were formulated and studied before fi nalizing the most probable one, from the effect of a bomb to the settlement of the soil caused by a sudden rise of the water-table, to the effect of air pollution, to the traffi c vibration and so on. Several documents were collected concerning the sudden collapse of other tow- ers even before the San Marco tower failure and the results of the investigation were interesting. In fact, the failure of some towers apparently happened a few years after a relatively low intensity shock took place. In other cases, the collapse took place after the development of signs of damage, such as some crack patterns, for a long time. This suggests that some phenomena developing over time had prob- ably to be involved in the causes of the failure, combined in a complex synergetic way with other factors. As for the experimental investigation carried out on some prisms cut out from the large blocks of the collapsed walls of the Pavia tower found on the site, the WIT Transactions on State of the Art in Science and Engineering, Vol 11, © 2007 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) doi:10.2495/978-1-84564-057-6/01 2 Learning from Failure attention was more and more concentrated on the dilatancy of the masonry under compressive monotonic and creep tests and on the fatigue behaviour of masonry under cycling loads. This chapter discusses the investigation carried out on the materials of the Civic Tower of Pavia and the conclusion reached by the previously mentioned Commit- tee. Furthermore, the phenomena of early and retarded deformations of historic masonry structures will be described together with the results of an investigation carried out on other damaged structures. Finally the research campaign carried out on site and in laboratory on the bell- tower of the Cathedral of Monza and the bell-tower of the Cathedral of Cremona. The investigation shows that the damaged state of the structures or of structural elements can be precociously detected by the recognition of the typical crack pat- terns, based on simple visual investigation. Collapses may be prevented by detecting the symptoms of structural decay, par- ticularly the crack patterns, through on-site survey, monitoring the structure move- ments for long enough periods of time, choosing appropriate analytical models and appropriate techniques for repair and strengthening the structures at recog- nized risk of failure. 1.2 The collapse of the Civic Tower of Pavia: search for the cause The Civic Tower of Pavia, an eleventh-century tower apparently made of brickwork masonry, suddenly collapsed on 17 March 1989 (Fig. 1.1). Several hypotheses were Figure 1.1: The ruins after the collapse, seen from the arcade opposite to the Cathedral. WIT Transactions on State of the Art in Science and Engineering, Vol 11, © 2007 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) Failures due to Long-Term Behaviour of Heavy Structures 3 made about the causes of that sudden failure, from soil settlements to the presence of a bomb, from vibrations caused by traffi c to the passage of super sonic jets. For a thorough understanding of the real causes of the collapse, an experimental investigation was carried out on site and in the laboratory, on the large amount of material coming from the remains of the tower. 1.2.1 Description and historic evolution of the tower The tower, about 60 m high with a square base measuring 12.3 × 12.3 m was located close to the north-west corner of the Cathedral. Each of the four facades was divided horizontally into six orders (Fig. 1.2a and b). The fi rst four from the bottom were divided into fi ve parts by four pilaster strips topped by two small arches. The third and fourth orders had no pilaster strips, but were topped by simi- lar hanging arches. The fi fth order terminated in a cornice. A large mullioned win- dow with two apertures opened out on each side of the sixteenth-century belfry. Inside the tower two timber fl oors were situated at a height of approximately 11 and 23 m. (a) (b) Figure 1.2: (a) The Civic Tower and Cathedral of Pavia, Italy. (b) Geometry of the Civic Tower and Cathedral of Pavia, Italy. WIT Transactions on State of the Art in Science and Engineering, Vol 11, © 2007 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) 4 Learning from Failure According to the few historical documents found, the fi rst order and half of the second order can be dated between 1060 and 1100 AD [1, 2], the part from the middle of the second order and the fi fth perhaps were built between the twelfth and thirteenth centuries; the tower was surmounted by a brick belfry and a timber roof. Between 1583 and 1598 the granite belfry weighing 3,000 tons, designed by the famous architect Pellegrino Tibaldi was set on top of the tower. A staircase built into the wall ran along all four walls from the south-west corner up to the belfry. The staircase was covered by a small barrel vault apparently made of conglomerate. 1.2.2 First experimental results and interpretation of the failure causes The few documents available at the time of the collapse [3] were insuffi cient to give an accurate geometric confi guration of the tower. Consequently, in order to draw prospects and sections of the tower the following operations, described in detail in [4, 5], were carried out: • topographic survey of the remains of the tower (Fig. 1.3), and partial rectifi ca- tion of existing photographs to defi ne the precise plan and the thickness and morphological features of the cross-section of the masonry; • reconstruction of the geometry of the belfry from a survey of the granite parts, practically all recovered from the internal portion of the remaining part of the tower; • assessment of the overall height of the tower from an existing aerial photogram- metric survey; • perspective plotting from existing photographs to reconstruct the geometry of the staircase and the arrangement of the architectural elements. 1.2.2.1 Structure and morphology of the walls The medieval walls, built according to the techniques normal at that time for tow- ers, were characterized by two external brick cladding ranging from 120 to 400 mm Figure 1.3: Photogrammetric survey of the remains of the tower. WIT Transactions on State of the Art in Science and Engineering, Vol 11, © 2007 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) Failures due to Long-Term Behaviour of Heavy Structures 5 with an average of 150 mm, with the intermediate portion of the walls consisting of irregular courses of large pebbles of brick and stones alternated with mortar, constituting a sort of conglomerate (Fig. 1.4). The walls of the second building phase were characterized by a much more irregular fi lling and by thinner external facings. Figure 1.5 shows one of the large blocks among the remains revealing part of the section of the wall with the external cladding. Figure 1.6 shows a complete cross-section of the wall of the present remains of the tower (south side), the ratio between the thickness of the external leaf of the wall and the internal one was approximately 1:16. The section of the wall near the staircase was composed by an external wall similar to the one described above, but 1400 mm thick, a stairwell 800 mm wide, and an internal wall 600 mm thick. The latter wall was of the rubble type and was particularly heterogeneous. Figure 1.4: Cross-section of the wall of the Civic Tower of Pavia. Figure 1.6: View of the complete section of the bearing wall. Note Figure 1.5: Part of the section of the how thin the external facing is in bearing wall (2.8 m thick), showing the comparison with the total thickness external brick cladding. of the wall. WIT Transactions on State of the Art in Science and Engineering, Vol 11, © 2007 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) 6 Learning from Failure 1.2.2.2 Geotechnical investigation The remains of the lower part of the tower left standing reached a height from 0.1 to 5 m still visible at the moment, since it was decided to leave the remains as they were without reconstructing the tower. The outside main wall is continuous and does not show any appreciable signs of dislocation or displacement from its original position. This, as well as the behaviour of the tower over time (there is no evidence of any specifi c surveys, but no appreciable settlement appears ever to have been reported), suggests that the collapse cannot be attributed to failure of the foundation soil.